SCENERY, SOIL AND THE ATMOSPHERE 577 



gard of latitude. These temperate and subtropical conditions in high 

 latitudes belong not only to ancient, but to the middle and modern ages 

 of the world, and geologists long ago surrendered the notion that they 

 could be due to supposed stores of the earth's primal heat. 



There have been periods of notable dryness, so that deposits of rock- 

 salt were formed through the evaporation of marine waters. From 

 New York westward occur beds of salt, due to a dry climate, in a region 

 where the rainfall is now abundant and where the basins of the Lauren- 

 tian lakes are filled to their brims. 



Not many years ago, the ice invasion of the Pleistocene was regarded 

 as simple in character and unique in time. "We now accept, among the 

 commonplaces of glacial geology, that the late ice invasion was com- 

 posite, marked by great advances and recessions, and by interglacial 

 times of genial climate. And we recognize further, among the accepted 

 facts of the science, the existence of vast glacial sheets in Permo-Car- 

 boniferous times, in India, Australia and South Africa, in regions 

 which are now either warm or warm-temperate, and in lands of no 

 great elevation. 



Yet more remote, in the Cambrian, in an age of early life forms, 

 an age recognized by the older geology as having almost ubiquitous 

 warmth, the evidences of extensive glaciation have been brought to 

 light. We must remember that humidity, precipitation, great or 

 slight, and variations of temperature, are intimate questions of meteor- 

 ology, whether we raise them in Cambrian or Miocene or present times. 

 The meteorology of the passing age is related to geologic climates 

 precisely as the physiography of existing lands is related to the rocks 

 and rock structures of the past. The atmosphere has undergone a 

 prolonged evolution in close association with the progress of the solid 

 earth. As a part of the earth's history the study of the atmosphere is 

 somewhat belated, but its importance is now recognized, and it will 

 fill a large place in the geology of the future. 



The present atmosphere therefore has not always existed and is but 

 the latest term in an evolutionary series. We find two leading assump- 

 tions concerning this history. 



There is a widely prevalent geological doctrine that our atmosphere 

 is a residuum from a more dense or a more extensive body of gases. 

 On this view it once contained all, or much, of the carbon dioxide 

 whose carbon is now wrapt up in the coal and the limestones of the 

 earth's crust. Thus Dana refers to the " purification of the air and 

 waters through the making of limestone " as commencing in later 

 Archean time and continuing through the Cambrian. 1 Accepting, as 

 he does, the idea that all the carbon of the coal and of many rocks was 

 originally in the air and the waters, he still finds difficulty, for in 

 early Paleozoic time life shows an atmosphere not too heavily charged 



1 " Manual of Geology," fourth ed., p. 484. 



